Quite often it's not so much tropes that I avoid rather than plot types that I don't think I am very good with. Anything that's action heavy is never going to be my forte, and neither is heavy angst. I do want to challenge myself in writing these genres a little more but I just think I'm not very good at doing it! For example I would love to write a fic loosely based on the movie Shelter (2007) but it would require enough angst that I keep on scaring myself from writing it.
I do plot my fics if they're going to be anything more than a 2-5k one shot. I tend to have a loose time line and points I want to hit as an outline, and when I think of lines for the story I add them there to use later as I tend to try and write in order (although I don't always). The few times I've not plotted much it took me literal years to finish the story so having an idea of where I'm going does help me, but I also can't have too much detail in the outline!
As for a ship... Lately I've been thinking a lot about humkeng but it's one of those where I'm like. I don't know I can do justice to the Kei in my head so I don't know that I ever will manage. But I'd like to one day!